1. Field of the Invention
This invention relates to a slip control system for a vehicle, and more particularly to a slip control system for a vehicle in which the driving wheels of the vehicle are prevented from slipping excessively by control of braking force.
2. Description of the Prior Art
Acceleration performance of a vehicle, especially during starting, is mainly governed by slip of the driving wheels relative to the road surface, and when the slip of the driving wheels exceeds a certain value, the acceleration performance of the vehicle largely deteriorates. Therefore, there have been proposed various systems for controlling the slip of the driving wheels to a proper level in order to keep an excellent acceleration performance. For example, as disclosed for instance in Japanese Unexamined Patent Publication No. 63(1988)-162359, there has been known a method of controlling slip of the driving wheels in which braking force applied to the driving wheels is controlled to control the driving torque of the driving wheels, thereby controlling the slip of the driving wheels to a proper level.
That is, in accordance with the method, a valve mechanism is provided in a hydraulic line which connects a master cylinder of the brake system and a brake provided for a driving wheel, and the valve mechanism is selectively moved between a position where it directly applies brake fluid from the master cylinder to the brake and a position where it returns brake fluid discharged from the brake to an oil reservoir, thereby controlling the driving torque of the driving wheel to converge the slip of the driving wheels to a proper level.
When the brake pedal is pushed down while such a slip control is being effected, the slip control is immediately interrupted and the brakes are applied preferentially.
When the brake fluid discharged from the brakes and returned to the oil reservoir is left there during the slip control, the amount of the brake fluid in the master cylinder reduces to cause excessive brake pedal travel.
In the conventional systems, in order to avoid such trouble, an oil pump is operated to return the brake fluid in the oil reservoir to the master cylinder when valve mechanism is operated to effect the slip control. Generally the oil pump is kept operated during the slip control and for a predetermined time after interruption of the slip control so that the brake fluid in the oil reservoir is returned to the master cylinder substantially completely.
However when the oil pump continues to return the brake fluid to the master cylinder after termination of the slip control in response to depression of the brake pedal, the brake pedal is kicked back to some extent under a counterforce of the brake fluid returned to the master cylinder and the kickback finely oscillates due to pulsation of the oil pump, which results in deterioration of braking feeling.